Smoke or fire detection systems which utilize a plurality of detectors or sensors spaced-apart in a region or area are known. One such system is disclosed in Tice et al. U.S. Pat. No. 4,916,432 entitled "Smoke And Fire Detection System Communication" which is assigned to the assignee of the present invention and which is incorporated herein by reference.
Known systems often provide a fixed alarm threshold at a control unit which is displaced from the sensors or detectors. The control unit communicates with the detectors or sensors via a bidirectional communication line of the type disclosed, for example, in the Tice et al. patent. Circuitry at the control unit senses a value or values returned from a selected detector or sensor which are indicative of a current ambient condition.
The sensed value or values is/are compared to a prestored threshold value which may be the same for all units. If the value or values returned from the selected detector or sensor exceed the prestored threshold value, the control unit makes a determination as to whether or not the system should go into an alarm condition.
An alarm condition can be indicated by an audible alarm. Alternately, an alarm condition can be indicated by a visual alarm.
Its recognized that the detector or sensor units vary in their behavior over a period of time after installation. Variations occur because of changing characteristics of electronic elements as they age, due to thermal stress for example. Variations also occur because different detectors are exposed to different ambient conditions.
Some detectors, for example, may be exposed to a very dusty environment. Other detectors, may be located in an area where there is a continual ambient smoke level due to normal conditions and not due to a dangerous smoke or fire condition. Additionally, some detectors or sensors may be located in an area with a higher continuous ambient temperature than other detectors thereby resulting in other variations.
These variations affect the value sent back by a given detector or sensor to the control panel. Hence, two detectors which are subjected to different environmental conditions, and which may age differently from one another, may send back to the control panel two different values indicative of the same non-smoke or clear air condition. Further, such detectors when placed into a test mode, may send back very different test values.
Thus, the known prior art practice of using a common predetermined threshold for all detectors has some serious drawbacks. It would be desirable to be able to determine a threshold for each detector, unique to that detector, which is based on the physical characteristics thereof as the detector ages. Further, it would be desirable to determine such a threshold remotely from the control panel without needing to make measurements at the detector or the sensor.
Finally, it would be desirable to be able to determine each detector's specific threshold on a periodic basis. Such periodically determined thresholds will more accurately reflect the aging or changing character of each of the detectors than will a fixed, unchangeable, common threshold.